The use of acetal copolymers for the production of lithographic offset printing plates is well known in the prior art because of their excellent film forming properties, good mechanical strength and superior chemical resistance on press. For example, U.S. Pat. Nos. 5,698,360 and 5,849,842 taught to prepare and utilize acetal copolymers containing sulfonamido functional groups as binder resins in UV photosensitive compositions used for conventional negative lithographic offset printing plates. Similarly, U.S. Pat. Nos. 5,925,491 and 5,985,996 taught that the use of acetal copolymers containing amido functional groups terminated with either hydrogen, C1-C8 saturated hydrocarbon, C1-C8 unsaturated hydrocarbon, or carboxylic acid functional groups as binder resins in UV photosensitive compositions leads to improved exposure and developing speeds. Furthermore, U.S. Pat. Nos. 6,087,066 and 6,270,938 taught that acetal copolymers containing meleinimido, funylvinylidene, thienylvinylidene and pyrrolyvinylidene functional groups used as binder resins in UV photosensitive compositions also lead to improved exposure and developing speeds. Also, U.S. Pat. Nos. 6,596,460 and 6,808,858 taught to prepare and use acetal copolymers containing azido, carboxylic acid or sulfonic acid functional groups as binder resins in UV photosensitive compositions to improve exposure and developing speeds.
Positive working lithographic offset printing plates containing near-infrared (NIR) laser radiation sensitive polymeric coatings are also known in the prior art. For example, Parsons, WO 9739894A1; Nagasaka, EP 0823327B1; Miyake, EP 0909657A2; West, WO 9842507A1; and Nguyen, WO 9911458A1 taught to prepare heat sensitive coating comprising a polymeric substance, a near-infrared absorbing compound and a dissolution inhibiting compound. In these coating compositions, the near-infrared absorbing and dissolution inhibiting compounds inhibit the polymeric substance from dissolving in the liquid developer by forming a network structure via hydrogen bonding or ionic interactions. Upon imaging with near-infrared laser light, this network structure is disrupted and thus, the exposed area becomes more soluble in the liquid developer, while the network structure of non-exposed areas is conserved and prevents the dissolution of this area (image area). However, the difference in solubility between the exposed and non-exposed areas varies during storage and usage, which makes these lithographic printing plates very difficult to process. For the printing plates that are just manufactured, the network structure in the coating composition is relatively weak and the non-exposed area is likely to be attacked by the liquid developer during processing, which leads to poor image quality. If the printing plates have been stored for some time, the network structure in the coating composition is very strong and makes it difficult to remove the laser-exposed area with the liquid developer. This phenomenon also leads to poor image quality of the printing products because of the background toning that occurs in such cases.
Different approaches have been taught in prior art to overcome the above-mentioned problems. For examples, U.S. Pat. No. 6,461,795 taught that, in order to accelerate the formation of a stable network structure within the coating composition, the lithographic printing plates must be heated at a preferable temperature between 50 and 60° C. in a low relative humidity atmosphere for several hours before shipment to the customers. Alternatively, U.S. Pat. No. 6,613,494 taught to apply a thin over-layer to protect the non-exposed area of the polymeric coating from the attack of the liquid developer.
U.S. Pat. No. 6,420,087 taught to prepare coating compositions for positive working lithographic printing plates containing siloxane compounds acting as image protecting agents that reduce the dissolution of the non-exposed areas during developing. However, the presence of these siloxane compounds made difficult the coating of the plates with roller coating techniques, caused phase separation in the coating solution and provoked the apparition of pinholes. In addition, these siloxane compounds are not soluble in alkaline developers, which causes sludge build-up in the processor, redeposit on the printing plates and a shortened lifetime of the developer.
WO patent application WO04020484A1 taught to prepare coating compositions consisting of acetal copolymers containing carboxylic acid, sulfonic acid and phosphoric acid terminated pendant groups, Novolak resin, near-infrared absorbing dyes, visible dyes, and image protecting agents for use in the production of thermally sensitive positive working lithographic offset printing plates having a high chemical resistance. Such coating compositions require a one-day post-production heat treatment at 50° C. in order to keep the image area from being attacked by the developer.
U.S. Pat. Nos. 6,255,033 and 6,541,181 taught to prepare acetal copolymers containing carboxylic acid, hydroxy, halide, methoxy and acetylene functional groups for use as binder resins in the production of positive working lithographic offset printing plates that can be imaged with near-infrared laser radiation. It is important to note that these coating compositions require an adhesion promoting agent, a near-infrared absorbing dye that converts light into heat and a large amount of visible acts as a dissolution inhibitor. In practice, high loading level of near-infrared dye and visible dye are required to differentiate exposed and non-exposed areas during development. However, the presence of such a large quantity of small organic molecules in the coating compositions reduces the mechanical strength of the coating, causes blooming during storage and severe staining of the processor during developing process after imaging.
U.S. Pat. Nos. 6,124,425 and 6,177,182 taught to prepare heat sensitive polymeric coating compositions for positive working lithographic printing plates comprising near-infrared absorbing chromophores grafted on the backbone of Novolak, acrylate or methacrylate based polymers. Optionally, these coating compositions may contain other binder resins and film-forming additives. Unfortunately, these coating compositions are difficult to functionalize, have only a limited mechanical strength, produce relatively short-lived plates and cannot be used with UV inks without baking.
Thus, there remains a need for new polymeric coating compositions for lithographic printing plates. The present invention seeks to meet these needs and other needs.